Miniaturization of electronic components has led to various improvements in semiconductor technology to make electronic components ever-smaller. Such components may include simple components such as diodes, up to complex components such as integrated circuits. Apart from electronic components, mechanical components can also be manufactured using the same technology.
In the art of semiconductor technology, it is commonly known that a wafer of semiconductor material, typically silicon, is processed to form the components in a surface area of the wafer. The wafer is macroscopic, having a diameter ranging in the order of 20-300 mm, while the components are microscopic, typically having a size in the micrometer range. Each component is made in a small wafer portion, with the various wafer portions being located at a small distance from each other. After the processing steps, the wafer is cut to separate the various wafer portions from each other, so that the components become available independent from each other. After separation, each separated wafer portion is referred to as a die, and the separation process is known as dicing. The present invention relates particularly to the field of wafer dicing.
The various wafer portions are typically arranged in a matrix pattern, separated by mutually orthogonal lanes, also indicated as “dicing streets”. The separation process involves applying a cut in each dicing street. Evidently, it is desirable that the surface area of the wafer is used as efficiently as possible, therefore said dicing streets are very narrow, which makes the precision requirements for the dicing processing very demanding. Further, along the said orthogonal lanes the top layer is an insulating or low-conductivity semiconductor material, which may be relatively brittle, and a traditional blade dicing method will cause severe damage to this top layer.
To overcome these problems, a hybrid dicing process has already been proposed in the prior art. This process is basically a two-step process, including a first step where radiation, typically a high power laser beam, is used to remove the top layer of the dicing streets, and a second step where a blade is used to cut the bulk silicon. The first step is also indicated as “radiative grooving”, or more conveniently as “laser grooving”. The present invention relates more particularly to a method of laser grooving.
FIG. 1 is a schematic top view of a portion of a wafer 1, showing component portions 3, wherein areas between the component portions 3 are indicated as dicing streets 4; these areas will be indicated as “dicing streets” in the situation when the grooving process has not been performed yet, but also in the situation when the grooving process has already been performed and the dicing street is hence provided with a groove.
FIG. 2 is a schematic cross section of a portion of the wafer 1, illustrating (on an exaggerated scale) subsequent steps in an exemplary prior art laser grooving process. The top layer of the wafer 1 is indicated at reference numeral 2. In a first step of the laser grooving process (see FIG. 1 righthand side, and FIG. 2 second picture), a relatively low power laser beam 11, 12 is directed to an edge area 13, 14 of a dicing street 4. Arrows indicate the relative movement of the laser beam 11, 12 and dicing street 4 with respect to each other, in a direction parallel to the longitudinal direction of the street 4. Laser power and beam speed are controlled such that the top region of the wafer 1 is removed (ablated) up to a relatively low depth and small width; the resulting elongate recesses at opposite sides of the streets 4 are indicated as “trenches” 15, 16. The depth of the trenches 15, 16 is larger than the thickness of the top layer 2.
In a second step of this exemplary laser grooving process (see FIG. 1 lefthand side, and FIG. 2 lower picture), a relatively high power laser beam 21 is directed to a central area 17 of the dicing street 4. The width of this laser beam 21 covers the entire street width between the trenches 15, 16. The resulting elongate central recess in the centre of the street 4 is indicated here as a “furrow” 18.
The combination of furrow 18 with adjacent trenches 15, 16 will be referred to collectively here as a groove 20. Depending on the precise process parameters, the individual furrow 18 and trenches 15, 16 may or may not be recognizable in the grooves 20. It is noted that FIG. 2 does not aim to provide an exact reproduction of the actual shape of the groove 2; especially along the side edges of the grooves, as material is typically raised above the undisturbed or original top surface of the wafer, forming a dike-like structure indicated as “burr”, although this is not shown in FIG. 2. The relative movement between the laser beam and the wafer may be practiced by holding the wafer stationary and moving the laser beam, or by holding the laser beam stationary and moving the wafer, or by moving both. In practice, it is more convenient to hold the optical system stationary and move the wafer; nevertheless, the movement will be indicated as a “scribing” movement of the laser beam with respect to the wafer. Thus, the first step of this exemplary laser grooving process may be referred to as “scribing” trenches, the second step may be referred to as “scribing” a furrow, and the overall process of forming a groove may be referred to as “scribing” a groove.
It is important to achieve a desired ablation profile having a relatively wide furrow with substantially constant depth over a large central part thereof. In practice, the high power laser beam 21 may consist of a matrix of high power laser beams 22, which together effect the material ablation up to the desired depth and width, because such matrix makes it more effective to achieve the desired profile.
Nevertheless, the entire process of scribing a groove involves many process parameters, including scribing speed and beam intensity, which all influence the groove profile achieved. If the groove profile deviates from the desired profile, later dicing steps may not achieve the desired separation and/or may lead to damaging the wafer. For example, if a groove is too shallow, it will still contain too much of the sensitive low-k material which will cause problems on cutting with the dicing blade; if a groove is not wide enough, problems may occur and proper separation may be hampered when the mechanical dicing blade touches the side edges of the groove. It is therefore important to check the quality of the grooves formed in the wafer.
According to the current state of the art, wafer inspection is performed in a separate wafer inspection apparatus. It is to be noted that the grooves typically have a depth in the range of 10 μm and a width in the range of 30-100 μm. For being able to inspect the grooves, an inspection resolution of 1 μm or better is required. This is not possible with a traditional light microscope, in any case not easily. In a typical prior art example, the inspection apparatus comprises a confocal microscope. Confocal microscopy is known to persons skilled in the art, and is not explained here in great detail. Typical for confocal microscopy is a very high optical resolution and contrast in the direction of the optical axis. By scanning the microscope in the direction of the optical axis, which is generally perpendicular to the wafer surface and indicated as Z-direction, there is a well-defined position where the groove bottom is in focus and hence visible. This represents the depth of the groove at that specific measuring position. Although this process gives accurate results, it has a serious drawback in that it is very time-consuming. Not only does scanning in the Z-direction take time before the accurate Z-value is obtained, it is further just one Z-value at one point. For obtaining a cross-profile it is necessary to repeat the measurement at multiple points. The inspection can therefore only be done on test samples, taken at random from the grooving machine. If the resulting grooves in the wafer are not according to specification, it takes a long time before the problem is noted and an operator can take action. By that time many more possibly incorrectly grooved wafers would have been produced which probably have to be discarded. Alternatively, one would have to halt the grooving machine during inspection of a previous wafer before grooving the next wafer, but this results in unacceptable process delays.